1. Field of the Invention
The present invention relates to a porous static-pressure air bearing. More specifically, it relates to a production method for producing a two-layered porous static-pressure air bearing with high rigidity and without dispersing the quality thereof.
2. Description of Related Art
Conventionally, static-pressure air bearing has been used for highly accurate sliding component of machine tools etc. The static-pressure air bearing blows gas such as air to the sliding component to support load between mutually sliding mechanical components by film of the gas, thus securing smooth slidability between the components.
Porous material has been used as a nozzle for blowing the gas as well as various known gas nozzle, since the porous material is suitable for the static-pressure air bearing because air jet can be equally distributed to a predetermined area.
In the above porous static-pressure air bearing using the porous body, a porous body having a bearing surface for blowing compressed gas such as air from the bearing surface to support the member to be supported and a base member for supporting the porous body have to be mutually bonded at a strength capable of sustaining air-supply pressure and have to be sealed so that the gas does not leak through the space between the porous body and the base member.
For obtaining the above adhesion strength and sealability, shrink fitting, internal chilling and screw fitting have been used for bonding the porous body to the base member. The shrink fitting has been practically used for bearing of machine tool spindle, where cylindrical porous graphite is shrink-fitted to steel base member. The internal chilling is made into practical use in a slider for linear guiding, in which ceramic mold slurry is immersed into porous foaming material composed of polyvinyl formal (PVF) etc. and the porous foaming material is internally chilled by the same ceramic mold slurry Japanese Patent Laid-Open Publication No. Hei 6-297421).
Further, the porous static-pressure air bearing has a vacancy only adjacent to the bearing side surface having smaller diameter than the diameter of the vacancy of a lower portion, for preventing generation of pneumatic hammer (self-induced vibration) caused by compressibility of gas at gas pool on terminal constriction or for excluding ununiformity of distribution of diameter and vacancy of the porous body to optimize flow rate of the compressed gas blown out from the bearing surface.
Conventionally, for narrowing the diameter of the hole adjacent to the bearing surface side, various methods are proposed, in which resin is immersed, plating is conducted or another porous layer having smaller vacancy diameter is provided to the bearing surface side of the porous body.
Incidentally, for attaching the porous body to the base member, since the shrink fitting uses frictional force for adhesion, great bonding force is difficult to be obtained unless the porous body has a large area for causing frictional force, in other words, unless the porous body has a configuration elongated in shrink fitting direction. Accordingly, the shrink fitting is not suitably used for thin shape object and thickness of the porous body to be fitted is restricted. The internal chilling has many complicated production process such as defoaming and drying, which results in longer production cycle and higher production cost. The screw fitting requires much trouble for machining the screw, which results in higher production cost, and the screw fitting is inferior in sealability.
For narrowing the diameter of the hole adjacent to the bearing surface side, immersion amount of the resin for obtaining the desired hole diameter and plating amount is difficult to be controlled in processes for immersing the resin and conducting the plating. When another porous material layer having smaller vacancy diameter is provided, the porous body as the base member is processed in a predetermined configuration and the other porous material layer is mounted thereon, thus complicating the production process.
An object of the present invention is to provide a production method of porous static-pressure air bearing for facilitating production of two-layered porous static-pressure air bearing and for securely bonding the porous body to the base member with great strength and with improved sealability.
For attaining the above object, the present invention includes following arrangement:
A production method of porous static-pressure air bearing having a bearing surface and a compressed gas supply surface opposite to the bearing surface according to the present invention includes following steps of: filling a through-hole provided on a base member with metal powder; bonding the metal powder with each other by thermal treatment and simultaneously bonding the metal powder to the base member to form a base porous body so that end surface on the bearing surface side of the metal powder filled in the through-hole forms a recess dented toward the compressed gas supply surface and at least a part of end surface on the compressed gas supply surface of the metal powder forms a recess dented toward the bearing surface; filling recess on the bearing surface with surface layer powder having smaller diameter than the metal powder; bonding the surface layer powder with each other and simultaneously bonding to the base porous body and the base member to form a surface porous material layer; and removing a face layer of the surface porous material layer by machining work to form the bearing surface.
According to the above production method, the base porous body can be formed only by filling the metal powder into the through-hole provided to the base member and by conducting thermal treatment to the metal powder. Further, since the base porous body is bonded to the base member metallurgically, the base porous body is strongly adhered to the base member and has good sealability. Further, since both of the gas supply surface and the bearing surface of the base porous body are formed into a predetermined shape simultaneously with being made into the porous body by the thermal treatment, no machining work is necessary for the base porous body. The surface porous material layer as the second layer is formed by filling the recess on the bearing surface with the surface layer powder having diameter smaller than the metal powder and by simultaneously bonding the surface layer powder with the base porous body and the base member. The surface porous material layer is made into the porous body in any manner in accordance with the material of the surface layer powder. Thermal treatment as well as bonding by the binder can be used in the same manner as the base porous body, thus obtaining sufficient bonding strength and sealability thereby. Further, only the face layer of the surface porous material layer is necessary to be machined.
The metal powder forming the base porous body may be at least one selected from the group consisting of at least one of bronze, brass and hard aluminum.
Solid lubricant bonded by a binder may preferably be used for the powder forming the surface porous material layer.
In the above arrangement, the face layer of the surface porous material layer may be machined by any one of grinding, lapping and carving with use of monocrystalline diamond bit.
The solid lubricant may be at least one selected from the group consisting of molybdenum disulfide, boron nitride and carbon.
Alternatively, the brittle material bonded by a binder may be used as the surface layer powder forming the surface porous material layer.
In the above arrangement, the face layer of the surface porous material layer may be machined by either one of grinding or lapping.
Ceramic may preferably be used as the brittle material.
A face layer of the base member may be removed simultaneously with removing the face layer of the surface porous material layer by machining.
Flow rate of compressed gas blown out from the bearing surface may be set at a predetermined level by controlling machining amount of the surface porous material layer.
The machining amount of the surface porous material layer may preferably be controlled by measuring the flow rate of the compressed gas blown out from the bearing surface through the surface porous material layer while supplying the compressed gas to a compressed gas supply surface side of the surface porous material layer through the base porous body simultaneously with machining.